Edm apparatus for finishing rolls

ABSTRACT

Rolling mill rolls, such as the final rolls for a sheet cold rolling operation, are machined by electrical discharges in a lathe-type machine with a roll length electrode tool. High areas of the roll shape the adjacent surface of the electrode tool, either by a mechanical or electrical wear-in operation, so that such surface substantially complements the desired final roll contour. The shaped electrode tool is subsequently used for electrical discharge machining (EDM) the roll to the level of the lowest roll portions so as to impart a very uniform matte texture to the roll surface and provide a roll with substantially perfect cross-sectional circularily. If the machining is carried out with a segmented electrode tool, the tool is preferably moved slightly from time-to-time axially of the roll during at least the final stages of the machining process. Following the machining, the roll may be used without further processing as an improved substitute for rolls textured by the conventional shot blasting technique or it may be ground to a bright polish.

United States Patent [191 Anderson [4 1 Mar. 26, 1974 EDM APPARATUS FORFINISHING ROLLS [75] Inventor: Alex Lennart Anderson, Rockford, 'm Truhe"L Assistant Exammer-Hugh D. Jaeger Attorney, Agent, or Firm-Wolfe,Hubbard, Leydig, [73] Assignee: The lngersoll Milling Machine Voit &Osann, Ltd.

Company, Rockford, Ill.

[22] Filed: Sept. 15, 1971 7 ABSTRACT PP N091 180,551 Rolling millrolls, such as the final rolls for a sheet Related u Application Datacold rolling operation, are machined by electrical discharges in alathe-type machine with a roll length elec- [63] g ggg sggfigggg oftrode tool. High areas of the roll shape the adjacent surface of theelectrode tool, either by a mechanical [52] us CL 219/69 V 51/49 219/69E or electrical wear-in operation, so that such surface 51/289substantially complements the desired final roll con- 51 Int. Cl 823k9/16 B24b 5/00 The Shaped electrode is subsequently used 58 Field ofSearch 51 /48-51 3 for electrical discharge machining (EDM) 51/53 103lO8 181 289 219/68 the level of the lowest roll portions so as to imparta 69 E M V very uniform matte texture to the roll surface and provide aroll with substantially perfect cross-sectional [56] References Citedcircularily. If the machining is carried out with a segmented electrodetool, the tool is preferably moved UNITED STATES PATENTS slightly fromtime-to-time axially of the roll during at 46l ,828 l89l least the finaltages of the machining process Follow- 2 156 ing the machining, the rollmay be used without furnoue l ther processing as an Improved substitutefor rolls tex- 3.240,9l4 3/1966 Hill et a]. 219/69 M tured y theConventional shot blasting technique or it FOREIGN PATENTS ORAPPLlCATlONS may be ground to a bright polish. 856,340 12/1960 GreatBritain 219/69 E l,076,094 7/1967 Great Britain 219/69 E Clam, 14 DrawmgFlgures 6O 1" r l PULSE PuLsE PULSE POWER POWER POWER SUPPLY SUPPLYSUPPLY 63 I l I I L 80 L 8 4: $72

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1 W 1 WWM rqfi'xl i %R dTToWEY-f 1 EDM APPARATUS FOR FINISHING ROLLSThis application is a continuation-in-part of my application Ser.No.78l,388 filed Dec. 5, i968 now abandoned, for EDM Method andApparatus for Finishing Rolls.

This invention relates to the finishing of rolls such as are employedfor rolling sheet metal and particularly to the finishing of hardenedsteel rolls used in rolling mills for final cold rolling of sheet steel.

The primary object of this invention is to improve the surfaceappearance of rolled sheet material by removing defects from the surfaceof the rolls employed in the rolling process.

Steel and aluminum sheet are examples of sheet materials requiringcareful attention to their surface. The sheet surface may desirably beara bright smooth appearance, in which event at least the last roll in themill is itself finished to a bright polish. Alternatively, the sheetsurface may desirably have a matte or pitted finish, in which event atleast the last roll in the mill is provided with a complementary mattesurface. In both cases, a particular problem has been the elimination ofappearance defects due to large area undulations caused by extremelysmall and gradual variations in the thickness of the sheet. Suchvariations are not readily detected by measuring devices in any smallarea of the sheet, but they may be visually noticeable under'certainlighting conditions in larger sheet expanses because of their adverseeffect on the planarity of the sheet surface. Such variations may alsobe the cause of burnishing or other surface appearance changes of thethicker portions of the sheet when it is pressed into a die cavity inmaking auto body parts or other products.

The problem of preventing departures from planarity in rolled sheetssurfaces is not a new one and has been under attack for many years. Forexample, it is known that the roll drive must be smooth and the rollbearings must be carefully aligned to avoid vibrations and pounding ofthe sheet as it is rolled. Rolling pressures are also calculated, and ifthere would be any tendency for the roll to defect under the rollingpressure selected, one or more crowns (roll segments of graduallyincreasing diameters) may be machined into the roll to offset thedeflection and thus eliminate it as a cause of any non-planarity. Thebest practices are followed in machining the roll to precise dimension,such as by finishing grinding the formed roll after hardening with alathe-type traveling head or traveling table precision grinder in anattempt to remove dimensional variations.

The small undulating defects I am concerned with are those remainingeven after the other practices designed to remove them or theireffect'have been followed. 1 have found that these small, butdistractingly observable, thickness variations are caused by departuresfrom roll cylindricity or cross-sectional circularity. In one form, suchdepartures are low areas distributed axially and circumferentially ofthe roll. These departures appear to be an almost inevitable result ofthe high reaction force between the roll and grinding tool employedduring the machining of the roll. In a lathe-type precision grinder, forexample, the powered wheel may tend to dig in at one edge during itstraverse along the roll and thereby leave 'a helical low area pattern onthe roll.

It is therefore another primary object of my invention to provide amethod and apparatus for finishing rolls to perfect cross-sectionalcircularity, by which 1 mean the elimination of axially andcircumferentially distributed low radius roll areas sufficient togenerate detectable defects in sheet produced by the roll.

Because of the high reaction forces involved in the conventionallyemployed grinding process, any yield whatever in the grinder may causethe small degree of wheel misalignment that leads to the production ofthe undesired low radius areas on the roll. Varying degrees of hardnessof the roll cause the reaction forces to change during the grindingoperation and, therefore, compound the problem by leading to anon-uniform metal removal rate. Reduction of the reaction forces byreducing the depth of the cut or the carriage speed is an obviousexpedient for reducing the errors, but a substantial increase inmachining time is required to achieve even a small improvement.

Thus, a further object of this invention is to provide an economicalmethod and means for employing elec trical discharge machining toaccurately finish machine rolls.

in electrical discharge machining, reaction forces between the workpieceand electrode tool are negligible, even when the electrode andelectrical conditions are selected to achieve a maximum machining rate.EDM has not heretofore been economical for machining large cylindricalworkpieces, such as steel rolls, since much faster machining has beenattainable with other processes, such as grinding. l have found thatthis disparity may be substantially reduced, at least to the extent thatthe many advantages of EDM roll finishing make it an acceptablealternative. Instead of having the electrode tool traverse the roll asin grinding, I employ an elongated electrode tool which spans the lengthof the roll and which preferably comprises several segments in order topermit the simultaneous machining of several different roll areas. Theedge of the electrode tool adjacent the roll is shaped to complement thedesired roll contour. Such shaping would appear to present problems akinto those presented in machining a roll, but I have found that veryprecise shaping may be readily accomplished, either by an electrical ormechanical wearin process, using the imperfectly contoured roll as theshaping tool.

Hence, it is still a further object of this invention to provide anelectrical discharge machining process for finishing rolls wherein theelectrode tool is first shaped by the roll and then employed to machinethe roll to a desired contour.

The use of a segmented electrode tool materially reduces the timerequired to machine a roll, but tends to produce a roll surface withcircumferential stripe-like defects in substantial alignment with thecorners of the electrode segments. Such defects do not prevent thefinished roll from having perfect cross-sectional circularity, but theydo comprise deviations from the desired roll contour and may cause therolled sheet to have observable appearance defects in the nature oflines running along its length. This problem may, however, be overcomeby moving or oscillating the electrode tool axially of the roll duringat least the final few minutes of the machining process. The amount ofmovement should be limited, particularly if a crowned roll contour issought, since such movement may tend to introduce errors into theelectrode contour and cause excessive electrical erosion of theelectrode tool requiring that it be frequently replaced. I have foundthat a useful limit is to confine the movement of the electrode tool toa distance about equal to the distance along a line parallel to the rollaxis between the corresponding ends of two adjacent electrode segments.This limit permits sufficient movement of the electrode tool toeliminate any observable circumferential striping of the roll and yetprevents any material change in the contour or wear rate of theelectrode tool.

It is therefore yet another object of this invention to provide anelectrical discharge machining method and means for finishing rolls inwhich adverse effects of electrode wear are avoided as the machiningproceeds.

Rolls have heretofore been finished with a matte texture to impart sucha texture to rolled sheets. Such a sheet texture may be useful prior tothe final rolling stage, say, to enable stacked sheets to be morereadily separated after annealing. Further, as a final surface finish, amatte texture is preferable to a smooth finish if the paint retainingcapabilities of the sheet are of concern. The conventional technique formatte texturing rolls is to use a shot blasting process. The shot orgrit is selected for size and shape and then propelled or blastedagainst the roll in an attempt to produce a uniformly pitted andunpeened surface. This objective has not, however, been fully realized.The matte surface provided by shot blasting is not uniform. Moreover,even if the roll has a high quality surface finish, the full advantagesthereof are often lost because of the aforementioned departures of theroll from perfect cross-sectional circularity. For example, variationsin the thickness of the rolled sheet may cause the matte surface of thethicker sheet areas to be selectively peened or destroyed when pressedinto a die cavity. On the other hand, electrical discharge machining ofthe roll not only provides the perfect roll cross-sectional circularitynecessary to avoid variations in the thickness of the rolled sheet, butalso imparts a very uniform matte surface to the roll. The dischargeenergy content may be adjusted to provide matte surface textures rangingfrom the very fine to the very course, but once the energy leveladjustment is made, the size of the particles dislodged from the roll bysuccessive discharges does not materially change.

Consequently, another object of this invention is to provide a rollfinishing process and apparatus for imparting an improved matte textureto rolls and sheet rolled thereby.

In some instances, smoothly finished sheet is required and the mattetexture imparted to the rolls by the electrical discharge machining mustthen be eliminated. Such a sheet finish is desirable, for example, toenable the sheet to be more positively gripped if it is to be furtherprocessed by later smooth rolls. Thus, a further object is to providecombined grinding and roll finishing apparatus for machining rolls toperfect cross-sectional circularity, with either a matte textured orpolished surface.

Other advantages and objects of my invention will become apparent whenthe following detailed description of a preferred embodiment andmodifications thereof is read in connection with the accompanyingdrawings, in which:

FIG. I is a perspective view, partially cut away, of a two-high,single-stand temper mill in which a pair of rolls, such as may bemachined by the invention, are shown in the process of cold reducing asteel sheet.

FIG. 2 is a plan view of a lathe-type traveling head roll grindingmachine conventionally employed for precision grinding of hardened steelrolls for installations such as the stand of FIG. 1.

FIG. 3 is an enlarged fragmentary view of the grinding wheel-rollsurface interface in the machine of FIG. 2 illustrating in exaggerateddegree the tendency of a wheel edge to dig into the roll surface as thewheel carriage travels along the roll.

FIG. 4 is a side elevation of a roll finished by the grinding machine ofFIG. 2 to show by shading the opposed helix low area pattern resultingfrom the edge grinding effect.

FIG. 5 is a greatly enlarged fragmentary view of a profile of a portionof the surface of the roll of FIG. 4 relative to a maximum radiusreference line.

FIG. 6 is a plan view of a single-position combined EDM and finishgrinding machine for perfect circle roll finishing.

FIG. 7 is a plan view of a two-position combined EDM and finish grindingmachine for perfect circle roll finishing.

FIG. 8 is a view, partially in section, of an apparatus for dischargemachining the surface of a roll to perfect circularity.

FIG. 9 is a semi-schematic perspective view of the electric andhydraulic circuitry associated with the FIG. 8 apparatus.

FIG. 10 is a plan view of a portion of the single-row electrode array ofthe FIG. 8 apparatus.

FIG. 11 is a plan view of a modified form of the FIG. 10 apparatusillustrating the use ofa two-row electrode array.

FIG. 12 is a side elevation ofa roll finished by electrical dischargemachining with the apparatus of FIG. 8 to show by shading thecircumferential stripe-like-defect pattern which may appear when astationary segmented electrode tool is employed.

FIG. 13 is a view generally corresponding to FIG. 8, but showing amodification made to oscillate the electrode tool so as to preventobservable roll defects of the type shown in FIG. 12.

FIG. 14 is a section, partly in phantom, of the modified apparatus, thesection being taken along the line l414 shown in FIG. 13.

While the invention is described in detail with reference to certainillustrated embodiments, it is to be understood that my intent is not tolimit it to those embodiments. To the contrary, I intend to cover allmodifications, alternatives, and equivalents falling within the spiritand scope of the invention as defined by the appended claims.

Referring now to FIG. 1, an exemplary two-high single-stand temper mill20, such as may be employed for the last step in a cold reductionprocess in making sheet steel, is illustrated. The pair of rolls 21 and22 are power driven to cold reduce the steel sheet 23 as the sheet isrolled between them. The dimensions are those conventionally associatedwith rolling wide sheets, the diameter of the working surface of each ofthe illustrated rolls being approximately 2 feet and the axial length ofeach working surface or body being approximately 8 feet. The roll issuitably crowned, as is not uncommon in a temper mill, the roll diametergradually increasing from the ends toward the center of the workingsurface in this particular instance to provide a single crown with acenter diameter 0.003 inches greater than the end diameters. The'crownprofile is designed to offset the deflection of the roll axis under therolling'reaction forces so that the roll surface along the path ofcontact with the sheet remains exactly parallel to the roll axis whilethe sheet is rolled. The number of crowns and the amount of any suchcrowning designed into the roll depends upon roll size, pressures andother factors. The problem to which my invention is addressed is thesame for rolls with different types and degrees of crowning or with nocrowning at all. Since the desired profile of the roll is very close toa straight line and is intended to be straight when rolling, l haveherein referred to a desired straight line profile of the roll withoutintent of suggesting that the invention is limited to uncrowned or flatrolls.

While perfect cross-sectional circularity (sometimes referred to hereinsimply as circularity or perfect circle finish) is theoreticallydesirable for any roll in the mill, the small departures fromcircularity caused by conventional roll finishing processes are toominute to be of concern in any but the last one or two stages of a coldrolling mill. Very planar sheet steel may be obtained using perfectcircle rolls in a final cold reduction stages to reduce the thickness orgauge of the sheet by approximately 1 1 /2 percent. At the present stateof the art, the formation of scale during hot rolling and the necessityof scale removal when appearance is a cominant consideration limit theadvantages to be gained by employing perfect circle finishing for therolls used in such a process.

Apart from any perfect circle finishing of the rolls 21 and 22 inaccordance with this invention, the rolls and the stand shown in FIG. 1are of conventional design. Thus only the working surfaces 24 of therolls need have perfect circularity because the other surfaces do notwork the metal. Indeed, the last two inches or so at the outer ends ofthe working surfaces also need not necessarily have perfect circularitysince the roll length generally exceeds the sheet or coil width by atleast several inches. The necks 25 at the outer ends of the workingsurfaces are usually one-step or two-step reduced cross-section diameterportions journalled in babbit bearings or chocks 26. The wobblers 27 atends of the rolls have flats or cut-away portions for positivecouplings. One end of each roll is usually connected through a couplingbox 28 to a spindle 29 and then through another coupling box 30 to theshaft of the mill drive motor 31.

For an appreciation of the appearance defect problem and the validity ofmy solution to it, reference is next made to a conventional machine asshown in FIG. 2 for finishing or refinishing the hardened steel workingrolls. This is a traveling head precision grinder 33 having a lathe-typebed with a roll 34 axially positioned by centers of the headstock 35 andthe tailstock 36. The headstock has a powered'spindle to rotate theroll, a drive collar 37 being suitably employed to engage the rollWobbler at the headstock end. Also shown are bearing supports 38 forsupporting the roll for rotation at the neck bearing surfaces designedfor the chocks. These supports are suitably of an adjustable three-pointbearing type. When grinding on centers with the cen ters designed tosupport the roll as well as carry the end thrust, the supports 38 neednot be employed. A pair of guide ways 39 on the machine base alongsidethe roll support the traveling carriage 40. A motor driven spindle 41 onthe carriage carries a rotary grinding wheel 42. The carriage also hascross slides (not shown) for feeding the grinding wheel toward the roll.ln operation, the roll is rotated in one direction by the machineheadstock, the grinding wheel is rotated to provide a differentialvelocity (usually by rotation in the other direction), and the carriagecarries the wheel past the dressing point and down the length of theroll and back with a given feed setting. Typically the wheel is fedtoward the roll for successively lighter cuts and at slower carriagespeeds in succeeding passes.

Since the sheet appearance depends primarily on the roll surfaces, eachroll must also be refinished from time to time due to spalling, marringor otherwise uneven wear of its surface during use. In addition, in installations where the roll has a matte surface for imprinting a mattesurface in the rolled product, the relatively rapid wear of the mattesurface may make it necessary to more frequently schedule the roll formatte finish renewal.

Even though the FIG. 2 grinder is carefully constructed to prevent theobvious sources of grinding inaccuracies, such as eccentric mounting ofthe roll or insufficient rotational support for the roll, minorfinishing inaccuracies may persist. These show up in the circumferential run-out (the difference between the maximum and minimumradii of the roll). Thus, a dial gauge having a sensing stylus pressedagainst the roll will indicate variations in radius as the roll isturned through one revolution. In various hardened steel rolls finishedby precision grinding machines, I have found a measurable run-out ateach circumference tested at distributed points along the length of theroll. While the maximum run-out on rolls ground by modern equipment isusually very small, say, about 0.0001 0.0002 of an inch as compared tothe roll diameter of, say, two feet, it is in fact large enough tocreate wave-like appearance defects in the rolled sheet. Furthermore,run-outs up to 0.0005 of an inch are not uncommon in rolls ground onolder machines. While eccentric mounting of the roll will alsocontribute to run-out, the run-out here referred to is that remainingafter eccentricity has been eliminated viz., the run-out attributable tohigh and low radius areas located at different angular positions inspaced circumferences along the length of the roll. The defect patterndetected is one in which the low areas (the areas of radii less than thedesign radius) are distributed axially and circumferentially of theroll. Thus, around the circumference of the roll taken at any pointalong its length, and along the length of the roll from any point on itscircumference, there are low radius areas deviating from the maximum ordesign radius for the roll.

This characteristic circumferentially and axially spaced defect patternappears to be caused by uneven or non-uniform attack of the roll surfaceby the grinding wheel. As shown in FIG. 3, the high reaction forcesbetween the roll surface and the traveling wheel 42 causes the wheel totilt slightly so that its leading edges cuts a continuous low area path44 into the roll. The degree of tilt and the sharpness of the wheel edgehave been exaggerated to facilitate illustration and it is, therefore tobe understood that the usual low area path profile can be expected tohave a more gradually curved departure from the design contour. Variouscombinations of looseness of the grinding wheel spindle in its bearingsand other factors causing play or yield in the grinding system defeat auniform cut by the entire flat peripheral surface of the wheel. The lowradius path generated in the roll around and along its length during asingle pass of the wheel is a continuous helix. A reverse helix low areapath is traced in the return pass of the grinding carriage. Thisopposing helix or criss-cross low area pattern leaves diamond-shapedhigh areas on the roll surface. These high areas are further divided andsubdivided by the succeeding passes of the wheel, but they are seldomcompletely removed.

The crossed helix low area pattern shown in FIG. 4 represents the defectpattern caused by the first forward and return pass and is consistentwith the run-out measurement pattern and other observations. The angleor pitch of the successive helical patterns on any given roll usuallydecreases, since the traverse speed of the grinding wheel is generallyreduced for succeeding finer finish passes. Thus, the different helicalpaths generated during the grinding of a roll with the dimensions of therolls shown, for example, in FIG. 1 are more or less randomly spacedfrom each other with the result that the complete cross-helix pattern islikely to be obscured. Usually the most noticeable parts of the patternare the apexes or circumferentially aligned V-shaped high areasoccurring where helical paths in reverse directions cross each other.These apexes typically repeat at intervals of 2 inches or less along thelength of the roll at any circumferential point. The cross-helix patternmay be complicated somewhat by the previous history of the roll becausepath variations in surface hardness and chatter marks or other machiningirregularities that may be present when finish grinding is begun usuallyaffect any inherent tendency of the wheel to dig in or yield.

A characteristic of the roll defect pattern which I have discovered tobe useful in removing the defect in the rolls is illustrated in part byFIG. 5 which shows in exaggerated detail the profile of the roll surfacein a low area as contrasted with the line of maximum radius run-out,which is the locus of maximum radius points at each circumference takenat each point along the length of the roll axis. If the roll is designedas a straight cylinder, the maximum run-out line is a straight line. Ifa crowned profile is called for, the line of maximum run-out has thecorresponding crown contour. In any event, the maximum radius is notless than the design radius at any circumference along the length of theroll, despite the crossed-helix grooves.

Thus, the low areas can be simultaneously detected and displayed byremoving the high or maximum radius areas by successive amounts as theroll turns on its axis. 1 have found that this is most readily done byelectrical discharge machining in which discharges from a tool electrodeconforming to the complement of the desired roll profile, whether it becrowned or straight, selectively machine the nearest portions of theroll as the roll rotates. In such a process, often called sparkmachining and which itself is old in the art, over-voltage initiateddischarges are repeatedly initiated across a narrow, dielectricliquid-filled gap defined between the workpiece electrode and theelectrode tool. Upon each application of a voltage impulse (assuming thegap spacing is within the usual operative limits) the discharge willgenerally occur in the specific region where the gap is narrowest.Particles are removed from both the workpiece and the tool by eachdischarge even though there is no measurable reaction force betweenthem, and the machining conditions are therefore selected to provide asuitably high ratio of workpiece removal to too] removal. The removal ofa very small workpiece particle by each discharge to form a microscopicpit in the workpiece increases the gap in that region by the removalamount, and leaves the next most narrow portion of the gap as the mostlikely site of the next discharge. Thus, by using a electrode toolshaped to complement the desired roll contour for machining a roll inaccordance with this invention, the high areas of the rotating roll aredifferentially removed without any measurable reaction force between thetool and roll electrodes.

Since a surface machined by spark or discharge machining ischaracteristically pitted and readily distinguished from a cut orabraded surface, the contrast between the machined and non-machinedportions of the roll makes visual inspection simple and more informativethan run-out measurements. Of course, as the discharge machiningproceeds,.the machined or former high areas become larger in proportionto the remaining untouched or non-machined areas. When the entire rollsurface can be seen to have been discharged machined, the roll iscompletely machined to perfect circularity i.e., the dimensiondepartures sufficient to produce appearance defects in the sheetproduced by the roll have been removed. By inspection of the rollsurface before completion, however, the extent and cause of defects maybe identified so that remedial measures can be taken. For example,dominant helical low areas of one pitch rather than another may indicatethe desirability of changing the grinding program. Chatter marks causedby an out-of-balance grinding wheel may also be readily identified.

As a final finishing process, electrical discharge machining of a rollnot only provides perfect roll circularity, but also imparts a veryuniform and easily controlled matte texture to the roll surface. Once alayer of surface metal has been removed by electrical dischargemachining, the roll surface texture, does not therefore change asadditional metal is removed by further machining. Unlike a shot blastedsurface, the EDM surface texture is" nearly uniform in terms of bothsize and distribution of pits. Thus, the paint retaining qualities ofsuch surface as transferred to the sheet are uniformly good and theappearance of the surface after painting is uniformly pleasing. Sincethe dimensions of the individual pits are on the order of micro-inches,the matte texture of the roll does not adversely affect the planarappearance of the rolled sheet. Various specifications for texturecoarseness in therolled product are met by controlling the energycontent of the discharges to establish the desired pit size, lowercurrent and shorter discharges resulting in smaller pits for finertextures.

Electrical discharge machining of the roll is also useful as anintermediate step when the roll is to be later ground to a polishedfinish. The removal of the departures from circularity by the dischargemachining step permits finish grinding to a high polish starting with avery light cut and slow traverse speed so as to materially reduce thetendency for the grinding wheel to skew and produce pronounced low areapatterns. Discharge machining to remove the undulating contour producedby prior rough grinding also eliminates any possibility that suchcontour may be perpetuated during the subsequent final cuts and,therefore, may be employed as an improved substitute for several of thecustomarily employed intermediate grinding steps. In some instances,rather than a roughv grinding-discharge machining-fine grindingsequence, the discharge machining step may be followed by a looseabrasive polishing step sufficient to smooth the discharge-machinedmatte surface to the desired degree of polish. As previously mentioned,at the present state of the art, substantial material removed bydischarge machining is usually undesirably slow. However, it should beappreciated that after rough grinding of a roll, there is only arelatively small volume of material that need be removed from the highareas to achieve the desired cross-sectional circularity. Of course,time permitting, the grinding 'EDM process may be employed without priorgrinding of the roll. Indeed, the preliminary grinding step ispreferably avoided whenever possible in the interest of increasing theuseful life of the roll by minimizing the amount of metal removal duringthe finishing or refinishing processes. For example, prior grinding cansometimes be advantageously bypassed in the refinishing of rolls withsurfaces free of deep gouges or other marks.

FIG. 6 illustrates one form of combining precision grinding and EDMmachining in which a roll 45 is mounted for rotation between theheadstock and tailstock centers 46 and 47 on a machine bed with a rotarygrinding wheel carriage 48 slidable on ways 49 along one side of thebed. An electric discharge machine electrode and dielectric liquidtrough assembly 50 is positioned under the roll, and the roll andelectrode are connected (by means not shown) to an EDM power supplysuitably mounted in a console 51 on the other side of the machine bed.Alternate grinding and discharge machining of the roll are facilitatedwithout the necessity of removing the roll from the machine. Thus, thisembodiment is particularly convenient where discharge machining is anintermediate step in the surfacing of the roll between rough and findgrinding steps.

FIG. 7 illustrates another form of combined precision grinding and EDMmachine which accommodates two rolls 52 in axial alignment. The rollsare supported endto-end by separate headstocks 53 and adjustablypositioned tailstocks 54. In this machine, a single grinding carriage 55slides on ways 56 extending parallel to the axis of both rolls so thatthe same grinder may machine either roll. An electric dischargemachining power supply 57 is connected to either of two similarelectrode tool and trough assemblies 58 which are positioned under therespective rolls. In normal use, one roll is ground while the other isdischarge machined, and each roll remains between the same centers forboth operations. This type of machining may be employed for either thetwo step grinding-discharge machining process or the three stepgrinding-discharge machininggrindingprocess. The grinding operation mayproceed on one roll while discharge machining proceeds on the other.Each of the separate roll drives 53 should have a very low speedavailable to facilitate inspection of the machining progress withoutstopping the roll.

In accordance with another aspect of the invention, the electrode toolis shaped to complement the exact desired contour of the roll by asimple wearing-in or setting process in which the shaping is done by thevery roll that is to be subsequently machined. The imperfectly contouredroll is a highly accurate tool for shaping the machining surface of theelectrode because, as

previously explained in connection with FIG. 5, the locus of maximumroll radius, which is generated during roll rotation, conforms to thedesign roll contour, whether it be straight or crowned. Thus, as theroll rotates, the proper configuration for the electrode surfaceopposite any given circumference of the roll is precisely defined by themaximum radius of the roll at the given circumference. Both mechanicaland electrical discharge processes for wearing-in the electrode can beadvantageously employed. In each process, the electrode whether it beone segment or an end-to-end array of shorter segments, is held in aholder aligned parallel to the roll axis and fed towards the roll axiswhile the roll is rotated. The process continues until the entire lengthof the machining surface of the electrode tool (the surface adjacent theroll) has been worn in. Obviously, the closer the electrode conformsbefore starting, the faster it can be worn-in or seated. An electrodecan be easily and inexpensively shaped to within a few hundreds orthousands of an inch to the roll design contour by conventional methods,but the problem solved in accordance with this aspect of the inventionis the precise machining and aligning of a long electrode or electrodearray.

In the mechanical wear-in process, which has thus far been found usefulonly for graphite electrodes (the conventional one of the non-metallicEDM electrode tool materials) the electrode array is held in an axiallyaligned position and fed radially to contact the rotating roll so thatthe electrode is abraded by rubbing. The graphite is sufficiently softand friable compared with the usual metallic roll materials not to scorethe roll and to be easily worn despite the relatively smooth surface ofthe roll. The holder for the electrodes and the feed system duringwear-in are advantageously those of the electric discharge system sothat the electrodes are abraded or worn in while in place for dischargemachining the same roll. After the electrode is worn in, the holder issimply backed off to define the machining gap to be maintained, anyloose particles are removed, and the discharge power supply is turnedon. If rolls of the same design contour are to be successively dischargemachined, the same electrode may be left in the holder and reused withlittle or no further wear-in necessary.

In the electrical wear-in process, the discharge machining operationeffective for machining the high areas of removed roll is simply acontinuation of the discharge machining initially effective for removingthe high areas of the tool electrode. This process is preferredgenerally over mechanical wear-in and is espe cially desirable forshaping electrode tools comprised of copper or other metals commonlyemployed in fabricating electrodes for electrical discharge machining.The relative removal rates of material from the roll and the electrodetool, together with the respective amounts of material rmoved therefromduring wear-in, largely determine the limits of accuracy. At thebeginning of the operation, the discharges occur between the high points(maximum radius areas) on the roll and the high points (areas extendingbeyond the desired complementary contour) of the tool electrode. Theelectrode is made relatively narrow in arcuate span so that itssubtended angle is very small compared to the composite angular extentof the high areas in the roll at any circumference. Thus, even if eachdischarge removes equal amounts of material from the electrode and thetool, the maximum radius run-out of the roll is not significantlyaltered during the electrode wear-in process because the high areas ofthe electrode are eroded long before the high areas of the roll. As aresult, the electrode is contoured to closely complement the maximumradius run-out of the rotating roll or, in other words, the desiredfinal roll contour. For example, with an electrode having an edgethickness of less than onehalf inch and with a roll having a diameter oftwo feet, the ratio of roll circumferential path to electrodecircumferential path is more than 150:1. While not all of the rollsurface at any given circumference is at the design radius due to thecircumferentially distributed low areas, the roll area is an order ororders of magnitude greater than the corresponding electrode tool area.Discharges to the roll low areas seldom, if ever, occur in view of thepreferential discharge to the axially and circumferentially distributedhigh areas adjacent the electrode at any instant, and hence the roll lowareas are not deepened in the course of wearing in the electrode too].After the electrode tool has been conformed to the maximum run-out orreference contour of the roll, the discharge machining process iscontinued to selectively remove the high areas of the roll. Theelectrode tool is itself eroded, but once having been worn in, the tollsurface wears uniformly to retain the desired contour as the roll ismachined As is conventional practice in discharge machining, the toolelectrode is positioned relative to the roll to maintain a gap spacingof a few thousandths of an inch as the machining proceeds. Automatic(i.e., servo control) of the tool feed is preferable in order to avoidshort circuits or current flow through gap debris. The impulse voltageis selected to be high enough so that a discharge is assured somewhereacross the gap. The exact location of the discharge, however, isinherently selective to occur with a high degree of probability at thethen-existing narrowest part of the gap.

For some electric discharge machining conditions the polarity of theelectrode relative to the workpiece has a large effect on theworkpiece-to-electrode wear ratio and the workpiece finish. Certaincombinations of electrodeand roll materials may yield a higher wearratio and/r a better roll surface finish if the machining is carried outwith the roll negative relative to the electrode (i.e., the so-calledreverse polarity condition). Others may yield superior results if themachining is carried out with the roll positive relative to theelectrode (i.e., the so-called normal or straight polarity condition).Thus, it is to be understood, that the relative polarities of theelectrode tool and roll may be selected for optimum performance. Whenappropriate for a particular combination of electrode and workpiecematerials, one polarity may be used for the electrode wear-in process toobtain a relatively low rolI-to-electrode wear ratio and the oppositepolarity may be used for the subsequent roll finishing to obtain arelatively high wear ratio. In that event, the polarity reversal orswitchover point is most conveniently determined by simply inspectingthe roll as the process continues since precise timing is not critical.When the cumulative axial width of the circumferential patches orstripes of textured areas on the roll approximates the axial width ofthe electrode (or, if segmented, the axial width of the segments), theelectrode can be assumed to have been worn in or fitted.

Segmenting of the electrode tool is preferred for faster machining. Thetotal tool electrode length must very nearly be equal to the axiallength of the working surface of the roll, but the discharge machiningof a roll with a long, say, 8 foot working surface is very slow iflimited by the rate at which discharge pulses can be supplied at thepresent state of the art using a single electrode 8 feet long. If,however, the electrode tool is segmented along its length, with eachsegment electrically insulated from the others and connected to aneffectively independent power supply, the machining rate can bemultiplied many times because several discharges may simultaneouslyoccur. The electrode segments must, of course, be rigidly held withrespect to each other to comprise an electrode array which is movable asa unit toward and away from the roll. The segments are preferably ofequal length so that no one segment is responsible for the machining ofsignificantly greater portions of the roll than any other segment. Thenumber of segments that may advantageously be employed depends primarilyon the number of effectively independent power supplies available and onthe number of segments that can be mechanically accommodated in an arrayhaving a length generally equal to the axial length of the particularroll that is to be machined. I have not noted any adverse effects oneither the overall contour of the electrode array or the cross-sectionalcircularity of the machined roll even when the discharge machining hasbeen carried out with effective segment lengths as short as about 0.8 ofan inch.

Additional details of a suitable mechanical arrangement of the roll andtool electrodes for discharge machining are illustrated by FIG. 8 wherea roll is shown positioned horizontally for rotation between centers.For rotating the roll over a range of speeds, including very low speedsto permit inspection of the defect pattern displayed from time to timeas the machining proceeds, a motor drive 61 engages one end of the roll.I have found peripheral speeds in the range of 150-300 feet per minute(about 25-50 r.p.m. with the two-foot diameter rolls of FIG. 1) to besatisfactory for discharge machining and lower speeds of, say, 2-5 rpm.to be satisfactory for inspection purposes. As shown, three pointbearing supports 62 support the weight of the roll at the roll necks,but the rolling mill chocks may themselves be employed as there islittle possibility that the particles removed by discharge machining canreach and contaminate the bearings. The tool electrode segments 63 arefixed in a rigid array to I a support beam 64 which, in turn, ispositioned under the roll and aligned parallel with the roll axis. Inthis embodiment, movement of the electrode array and support beam inother than the vertical direction is constrained by fixed verticalslideways 65 anchored to the machine bed or foundation. A tongue orslide 66 depending from the beam engages the ways 65 so that tilting ofthe beam 64 is also prevented.

A hydraulic cylinder 67 is suitably employed to set the verticalposition of the beam 64. The vertical extent of the slideways andcylinder length are largely dependent on the different roll diameters tobe accommodated by the equipment because very little vertical travel isrequired after the electrode to roll gap spacing has been initiallyapproximated. The cylinder body is suitably secured to the ways 65 andthe piston to a bracket 68 on the beam 64 for controlled lifting andlowering of the beam and electrode tool under hydraulic fluid flow. Anyconventional hydraulic system may be employed, and mechanical orelectrical lift systems may be substituted as desired.

To maintain good electrical contact with the roll, brushes 69 bearagainst a portion of the roll neck at each end of the roll forconnection to the common or ground terminal of the power supply orsupplies. As a safety measure, the entire roll support and drive ispreferably at ground potential, but the use of the brushes makes itunnecessary to rely on the usually higher resistance and inductance paththrough the drive or support bearing system. As will be seen, theelectrode segments are electrically insulated from each other. To thatend, each segment 63 is held in place against an insulating strip 70which lines the inner face of a shoulder 71 provided by the beam 64.This is suitably accomplished by means of insulated screws 72 which arethreaded into the segments and tightened to draw the segments againstthe insulating strip 70. The heads of the screws 72 are, of course,convenient terminals for electrical connections between the segments 63and a power supply. Further, the segments 63 are spaced apart so thatthere are narrow gaps between them (sufficient insulation is provided bya gap even as small as 0.005 of an inch) and underlying each segmentthere is an insulator spacer 73 which has a shoulder 74 to support thelower edge of the segment and thereby prevent it from skewing out ofline. The spacers 73 are preferably shorter than the segments 63 andpositioned so that they do not bridge the gaps between adjacent segmentsso as to simplify the flushing of any debris (particles removed from theelectrode or workpiece) which may sift into those gaps during themachining.

The machining rate depends in large part on the thickness of theelectrode because material may be removed from any given high area ofthe roll only when that area is closer to the electrode than any otherroll area. At the same time, however, the electrode must not be so thickas to prevent flushing of debris from the work gap because otherwise thedebris might tend to bridge the gap between the electrode withundesirable consequences, such as recutting (scoring) or burning of theroll. Preferably, the electrode thickness is uniform along the length ofthe roll in order to machine all circumferences of the roll atsubstantially the same rate. Thus, to maintain a uniform effectiveelectrode thickness with a segmented array, the ends of the electrodesegments 63 have complementary bevels and the bevel angles are selected(taking the inter-segment gaps into account) so that the rearwardcorners of each but one of the end segments align in the direction ofroll rotation with the forward corners of one of the adjacent segments.In the illustrated embodiment, the bevel angles are selected so that therearward corners of each segment align with the forward corners of thenext segment to the right and it is, therefore, the righthand endsegment which does not overlie any other segment. The result is that theeffective electrode thickness at any point along the array is acomposite comprised of a part of one segment and a part of anothersegment. I have found that satisfactory discharge machining is achievedwith an electrode thickness of about three-sixteenths to three-eighthsof an inch.

It is especially desirable to minimize the possibility of what is knowna gap short circuit condition (insufficient spacing between theelectrode tool and the roll most commonly caused by the effectivespacing be reduced by a build-up of debris) lest machiningirregularities be introduced by too frequent backing off of theelectrode tool to clear the condition. In the illustrated embodiment,the debris is flushed from the machining gap by the circulation of thedielectric liquid caused by the pumping action afforded by the rotatingroll. To retain the liquid, a trough 74a is provided around the array ofelectrode segments 63. For convenience, the trough side and end wallsare secured to the electrode support beam 64. The trough walls should behigh enough to permit the liquid level to rise above the bottom of theroll being machined and thereby assure constant inundation of the gap.Squeegee blades 75 may be fastened along the top of the trough sidewallsto re move the surplus liquid and debris from the roll surface as itleaves or enters the trough and to keep foreign matter out. Variousexpedients well known in the art may also be employed to aid inpreventing any build-up of debris in the gap, such as filtering andrecirculating the dielectric or directing jets of clean liquid to thesurface of the roll entering the gap. It will, of course, be appreciatedthat the electrodes may be placed in positions other than directly underthe roll if means are provided to supply and retain liquid in the gap.It will also be understood that when a segmented electrode array isused, the inter-segment gaps should at least roughly offer the sameresistance to flow of the dielectric liquid as the machining gap so thatall gaps are adequately flushed.

As further shown in FIG. 9 for the illustrated segmented electrode tool,each segment 63 is connected to one terminal of a separate pulse powersupply 80, the other terminal of each power supply being grounded andconnected to the brushes 69. Simultaneous voltage pulses of the desiredpolarity can thus be periodically applied across the gap between theroll and each segment. The required separation or isolation of the powersupplies is a matter of degree. Separate power supplies are effectivelyprovided using a single, low internal resistance, power source if thereis suffcient isolating resistance in the separate electrode switching orcoupling circuits to prevent a discharge at one electrode from droppingthe source voltage below the level necessary to initiate anotherdischarge at another electrode. The actuation of the hydraulic cylinder67 is preferably under an automatic electrohydraulic servo control 76responsive to the average gap voltage through a sensing network 77. Asis conventional where multiple power supplies are used with a commonservo, the average voltage measured is that for the segment at which theaverage gap voltage (and therefore the spacing) is least. The design ofthe electrical power supply and servo system are not themselves part ofmy invention, and various suitable electric discharge machining systemsmay be employed. One multiple electrode power supply system is shown,for example, in pending application Ser. No. 727, l 99 of Robert B.Bertolasi, filed May 7, 1968.

Since the thickness of the stock to be removed is very small, the servosystem is primarily useful during the electrode wear-in and initialmachining stages. lf desired hand regulated feed may be employed, butthe machining speed is usually reduced in doing so because of thelikelihood of short circuiting or conduction through gap debris,

Multiple rows of electrodes in the same electrode holder may be used asillustrated by the double rows 78 and 79 of segmented electrodes asshown in FIG. 11. This makes it possible to increase the obtainablemachining speed by increasing the number of separate power supplies inuse without further reducing the axial length of the individualelectrode segments.

The method and apparatus for roll finishing hereinabove described havebeen found to be very effective for finishing rolls to perfectcross-sectional circularity while imparting a superior matte texture tothe surface of such rolls. Thus, without more, the present invention isan improvement over the conventional roll finishing processes. I have,however, noted that at least some of the rolls finished in accordancewith this invention through the use of a segmented electrode array havefaint stripes which extend circumferentially of the roll in generalalignment with the corners of the segments. This pattern ofcircumferential stripes is shown by shading in FIG. 12 to sharplycontrast with the other roll areas, but such showing is exaggerated inthe interest of clarity, it being understood that the pattern is usuallyso faint that it can only occassionally be seen even upon carefulinspection of the roll surface. The nature and cause of the striping arenot completely understood. The stripes apparently represent a veryslight irregularity in the roll contour, and it is believed that theymay be low radius areas caused by the segment corners being preferentialarc terminii. At any rate, the pattern is undesireable because it isimprinted on the rolled product in the form of lengthwise extendinglines which may be visually detectable under some lighting conditions.

The striping pattern may be prevented or eliminated (i.e., at leastreduced so that its presence cannot be detected upon inspection of theroll or rolled product) without sacrificing the increased machiningefficiency afforded by the segmented electrode array. This may beaccomplished by using a segmented electrode array for dischargemachining the roll to perfect crosssectional circularity and by thenusing a continuous roll length electrode tool to further machine theroll until the striping pattern has been removed. The changeover fromthe segmented to the continuous electrode tool may be affected byremoving the segmented tool and installing the continuous tool in itsplace or, if each has a separate holder, simply by de-energizing thesegmented tool and energizing the continuous tool. The timing of thechangeover, is not critical, but a useful guide is to make thechangeover when the perfect circle finishing of the roll has beencompleted as indicated by the-uniform appearance of its working surface.The amount of time to be devoted to the machining with the continuouselectrode is not so easily defined because it depends on a number ofvariables, including the machining conditions, the roll and electrodecomposition and the severity of the roll striping pattern. Visualinspection of the roll surface is not completely reliable since thepattern-to be eliminated is very faint and may be obsecured by thecharacteristically pitted condition of the roll surface. Thus, theamount of time to be allotted for the continuous electrode machiningshould be conservatively estimated on the basis of experience gainedthrough, say, a cut and try technique.

I, however, prefer to avoid the increased complexity and additionalmachining time inherent in the use of a continuous electrode for finalmachining of the roll. I

have found that the roll striping pattern may also be eliminated bymoving the segmented electrode array in a direction parallel to the rollaxis from time-to-time during at least the final few minutes of themachining operation. There is a tendency for the contour of themachining edge of the electrode array to be altered whenever the arrayis shifted axially of the roll, but the movement necessary to eliminatethe striping pattern is so slight that there is no material alterationof the electrode edge contour, even if the roll is crowned.Specifically, the distance traversed by the array to eliminate thestriping pattern need be no greater than the distance axially of theroll between the forward corner at one end (say, the right-hand end) ofone segment and the rearward corner at the corresponding end (theright-hand end) of the adjacent segment. If the electrode array has auniform effective thickness, the above-identified distance is simply theaxial length of one segment.

The apparatus of FIG. 8 may be readily modified as illustrated in FIGS.12 and 13 to permit axial move ment of the electrode array. Little morethan means mounting the support beam 64 and the array fixed thereto formovement axially of the roll, such as the parallel, axially extendinghorizontal ways 10] and 102 shown, are required because the desiredmotion of the array may be provided manually, say, through the operationof a turnbuckle (not shown) connected between the support beam and afixed point, such as a steady rest 103. [t is, however, more convenientand less demanding on the operator if there is also a suitable drivemechanism for moving the array automatically in accordance with apredetermined program. Thus, in the illustrated embodiment, there is ahydraulic cylinder 104 which has its piston secured to the support beam64 and its body secured to the steady rest 103. The cylinder 104 ispressurized to bias the support beam 64 into engagement with aneccentric cam 105 which, in turn, is rotated by power supplied by amotor 106 through an appropriate speed reduction coupling 107. Thus, thesupport beam and electrode array therefore oscillate axially of the rollthrough a distance dependent on the difference between the maximum andminimum radii of the eccentric and at a rate dependent on the rotationalspeed of the eccentric. I prefer to limit the distance traversed by thearray to roughly the length of one segment. This is a generallysatisfactory limit when the segment is no more than a couple of inchesor so. For arrays with longer segments, a more restrictive limit may benecessary to insure that the portion of the roll working surface thatmay reasonably be anticipated to engage the sheet during the rollingoperation (all but the last 2-3 inches at either end) is uniformlymachined and to prevent the electrode edge contour from being materiallyaltered. Hence, as an alternative limit for arrays comprised of longsegments, it is worth noting that the pattern of circumferential stripeshas not been detected on rolls discharge machined using an electrodearray having segments 8 inches long by three-eighths of an inch thickbeveled at a 45 angle, when the array has been re-positioned every 3-5minutes within a limited axial span of threeeighths of an inch. l havefound that the axial movement of the array need not be continuous andthat the striping pattern is eliminated even if the average rate ofmovement is as slow as about 0.015 of an inch per minute.

17 Turning to the more detailed aspects of the embodiment illustrated byFIGS. 13 and 14, it will be seen that the motor 106 and speed reductioncoupling 107 for the eccentric 105 are mounted on a carriage 108 whichis releasably anchored in position on the ways 101 and 102 by manuallyoperated clamps 111 and 112. The bearing support 1 13 for the end of theroll opposite the steady rest 103 is also mounted (by means not shown)to be repositioned axially of the roll. Thus, the equipment has theflexibility necessary to accommodate a variety of different rolllengths. For set-up purposes, the positions of the bearing support 113and the support beam 64 are adjusted for the given roll length. Thecarriage 108 is then advanced until the eccentric 105 engages thesupport beam 64. The clamps 111 and 112 are tightened and pressure isthen applied to the clinder 104 to bias the support beam against theeccentric. An electrode array and trough sized to the given roll lengthare installed and the roll is set in place. The set-up is then completeand the discharge machining may proceed.

ln summary, it will now be understood that my invention provides animproved method and means for finishing rolls such as are used inrolling sheet steel. The

finished rolls are characterized by perfect crosssectional circularityand a very uniform matte surface texture, even if the hardness of theroll surface varies from point-to-point. The matte texture may, ifdesired, be removed by polishing the roll surface. It will also beappreciated that the improved roll characteristics result from EDMfinishing of the roll using a roll-length electrode tool, which may becontoured by the imperfectly contoured roll to complement the desiredroll contour. Finally, it will be understood that the electrode tool maybe segmented for relative fast machining of the roll and that thesegmented tool may be oscillated or otherwise moved from time-to-timeaxially of the roll through a limited distance to optimize the contourof the finished roll.

I claim as my invention:

1. Apparatus for finishing an elongated working surface of a rollsubject to axially and circumferentially distributed low radius defectareas to an ultimately desired design roll contour of substantiallyperfect crosssection circularity including means for rotating the rollabout its longitudinal axis;

a narrow electrode tool having a length substantially equal that of theroll working surface and a contoured longitudinal edge substantiallycomplementing a desired contour for the roll working surface;

means for positioning said electrode tool parallel to the roll axis withsaid edge adjacent but slightly spaced from the roll working surface;

means coupled to the roll and electrode tool for supplying a series ofover-voltage initiated discharges therebetween to remove particles fromthe working surface area of the roll successively least distant from theelectrode surface as the roll rotates; and means coupled to saidelectrode tool for moving said tool toward the axis of said roll tomaintain a substantially constant gap between the electrode edge and theroll working surface as particles are removed from the working surfaceof the roll. 2. The apparatus of claim 1 wherein said electrode tool isdivided into a plurality of segments electrically insulated from eachother, and said means for supplying discharges includes means forsupplying discharges between each of said segments and said roll.

3. The apparatus of claim 2 wherein said segments are of equal lengthand have end portions beveled at complementary angles selected toprovide a uniform effective width for said electrode tool along itsentire length.

4. The apparatus of claim 2 further including means coupled to saidelectrode tool for oscillating the electrode tool in a directionparallel to the roll axis with the total traverse of the electrode toolbeing limited so as not to exceed the length of a single segment.

5. The apparatus of claim 2 wherein said working surface has a lengthselected to provide at each end of the roll a relatively short incrementwhich may be imperfectly finished, and further including means mountingsaid electrode tool for movement longitudinally of said roll over alimited distance shorter than each of said increments.

6. Apparatus for finishing an elongated working surface of a rollsubject to axially and circumferentially distributed low radius defectareas to an ultimately desired design roll contour of substantiallyperfect crosssectional circularity comprising means for rotating theroll on its longitudinal axis;

a narrow elongated electrode tool divided lengthwise to provide aplurality of electrically insulated segments and having a longitudinaledge of substantially uniform width in the direction of roll rotation atall points along the electrode too], said edge being shaped tocomplement a predetermined ultimately desired contour for said rollworking surface;

means for holding said electrode tool with said edge parallel to theroll axis but slightly spaced from the roll to define a gaptherebetween;

means for retaining a dielectric liquid between the electrode edge andthe roll working surface to flood said gap;

electrically isolated means coupled between said roll and said electrodetool for supplying a series of over-voltage initiated discharges betweeneach of said segments and said roll for removing particles from pointson said roll successively least distant from the electrode edge as theroll rotates; and

servo control means coupled to said electrode tool and responsive to asignal representative of the most closely spaced electrode and rollareas for moving said electrode tool to maintain a predetermined minimumgap spacing as particles are removed from the roll.

7. The apparatus of claim 6 wherein said electrode tool is also dividedwidth-wise to provide a plurality of rows of electrically insulatedsegements.

8. A combined grinding and discharge machining apparatus for finishingan elongated working surface of a roll subject to axially andcircumferentially distributed low radius defect areas to an ultimatelydesired design roll contour of substantially perfect cross-sectioncircularity comprising a machine bed with means for supporting a rollhorizontally for rotation;

means for rotating the roll about its longitudinal axis at a controlledspeed;

a carriage on the bed slidable along the length of the roll axis at oneside of the roll;

a grinding wheel on the carriage with its periphery adjustable forgrinding contact with the roll surface to finish said roll toapproximate cross-sectional circularity while leaving a generallyhelical pattern of axially and radially distributed high radius defectareas;

a narrow, elongated electrode tool having a contoured longitudinal edgecomplementing a desired contour for said roll working surface;

means for positioning the electrode tool adjacent the bottom of the rollsurface with its longitudinal axis parallel to the longitudinal axis ofsaid roll and with a maintained small gap spacing between said electrodetool and said roll; and

means for applying a series of over-voltage initiated particle-removingdischarges between the electrode tool and the roll to further finishsaid roll by removing particles from the working surface areas of theroll successively least distant from the electrode surface as the rollrotates.

9. The apparatus of claim 8 further including a trough stationed underthe roll position and positioned around the electrode tool for carryingdielectric liquid at a level sufficient to flood the gap between thetool and roll.

10. Apparatus for finishing an leongated elongated surface of a rollsubject to axially and circumferentially distributed low radius defectareas to an ultimately desired design roll contour of substantiallyperfect crosssection circularity including means for rotating the rollabout its longitudinal axis;

a narrow electrode tool having a length substantially equal that of theroll working surface and a contoured longitudinal edge substantiallycomplementing a desired contour for the roll working surface, saidelectrode tool being divided into a plurality of segments electricallyinsulated from each other and having their end portions complementarilybeveled at angles selected to cause each segment to be aligned in thedirection of roll rotation in overlapping relationship with an adjacentsegment so that the effective edge width of the electrode tool at anypoint along its length is comprised of parts of two adjacent segments;

means for positioning said electrode tool parallel to the roll axis withsaid edge adjacent but slightly spaced from the roll working surface;

means coupled to the roll and electrode tool for supplying a series ofover-voltage initiated discharges therebetween to remove particles fromthe working surface area of the roll successively least distant from theelectrode surface as the roll rotates, said means for supplyingdischarges including means for supplying discharges between each of saidsegments and said roll;

means coupled to said electrode tool for moving said tool toward theaxis of said roll to maintain a substantially constant gap between theelectrode edge and the roll working surface as particles are removedfrom the working surface of the roll; and

means for retaining a dielectric liquid between the electrode edge andthe roll working surface to flood said gap.

1. Apparatus for finishing an elongated working surface of a rollsubject to axially and circumferentially distributed low radius defectareas to an ultimately desired design roll contour of substantiallyperfect cross-section circularity including means for rotating the rollabout its longitudinal axis; a narrow electrode tool having a lengthsubstantially equal that of the roll working surface and a contouredlongitudinal edge substantially complementing a desired contour for theroll working surface; means for positioning said electrode tool parallelto the roll axis with said edge adjacent but slightly spaced from theroll working surface; means coupled to the roll and electrode tool forsupplying a series of over-voltage initiated discharges therebetween toremove particles from the working surface area of the roll successivelyleast distant from the electrode surface as the roll rotates; and meanscoupled to said electrode tool for moving said tool toward the axis ofsaid roll to maintain a substantially constant gap between the electrodeedge and the roll working surface as particles are removed from theworking surface of the roll.
 2. The apparatus of claim 1 wherein saidelectrode tool is divided into a plurality of segments electricallyinsulated from each other, and said means for supplying dischargesincludes means for supplying discharges between each of said segmentsand said roll.
 3. The apparatus of claim 2 wherein said segments are ofequal length and have end portions beveled at complementary anglesselected to provide a uniform effective width for said electrode toolalong its entire length.
 4. The apparatus of claim 2 further includingmeans coupled to said electrode tool for oscillating the electrode toolin a direction parallel to the roll axis with the total traverse of theelectrode tool being limited so as not to exceed the length of a singlesegment.
 5. The apparatus of claim 2 wherein said working surface has alength selected to provide at each end of the roll a relatively shortincrement which may be imperfectly finished, and further including meansmounting said electrode tool for movement longitudinally of said rollover a limited distance shorter than each of said increments. 6.Apparatus for finishing an elongated working surface of a roll subjectto axially and circumferentially distributed low radius defect areas toan ultimately desired design roll contour of substantially perfectcross-sectional circularity comprising means for rotating the roll onits longitudinal axis; a narrow elongated electrode tool dividedlengthwise to provide a plurality of electrically insulated segments andhaving a longitudinal edge of substantially uniform width in thedirection of roll rotation at all points along the electrode tool, saidedge being shaped to complement a predetermined ultimately desiredcontour for said roll working surface; means for holding said electrodetool with said edge parallel to the roll axis but slightly spaced fromthe roll to define a gap therebetween; means for retaining a dielectricliquid between the electrode edge and the roll working surface to floodsaid gap; electrically isolated means coupled between said roll and saidelectrode tool for supplying a series of over-voltage initiateddischarges between each of said segments and said roll for removingparticles from points on said roll successively least distant from theelectrode edge as the roll rotates; and servo control means coupled tosaid electrode tool and responsive to a signal representative of themost cloSely spaced electrode and roll areas for moving said electrodetool to maintain a predetermined minimum gap spacing as particles areremoved from the roll.
 7. The apparatus of claim 6 wherein saidelectrode tool is also divided width-wise to provide a plurality of rowsof electrically insulated segements.
 8. A combined grinding anddischarge machining apparatus for finishing an elongated working surfaceof a roll subject to axially and circumferentially distributed lowradius defect areas to an ultimately desired design roll contour ofsubstantially perfect cross-section circularity comprising a machine bedwith means for supporting a roll horizontally for rotation; means forrotating the roll about its longitudinal axis at a controlled speed; acarriage on the bed slidable along the length of the roll axis at oneside of the roll; a grinding wheel on the carriage with its peripheryadjustable for grinding contact with the roll surface to finish saidroll to approximate cross-sectional circularity while leaving agenerally helical pattern of axially and radially distributed highradius defect areas; a narrow, elongated electrode tool having acontoured longitudinal edge complementing a desired contour for saidroll working surface; means for positioning the electrode tool adjacentthe bottom of the roll surface with its longitudinal axis parallel tothe longitudinal axis of said roll and with a maintained small gapspacing between said electrode tool and said roll; and means forapplying a series of over-voltage initiated particle-removing dischargesbetween the electrode tool and the roll to further finish said roll byremoving particles from the working surface areas of the rollsuccessively least distant from the electrode surface as the rollrotates.
 9. The apparatus of claim 8 further including a troughstationed under the roll position and positioned around the electrodetool for carrying dielectric liquid at a level sufficient to flood thegap between the tool and roll.
 10. Apparatus for finishing an leongatedelongated surface of a roll subject to axially and circumferentiallydistributed low radius defect areas to an ultimately desired design rollcontour of substantially perfect cross-section circularity includingmeans for rotating the roll about its longitudinal axis; a narrowelectrode tool having a length substantially equal that of the rollworking surface and a contoured longitudinal edge substantiallycomplementing a desired contour for the roll working surface, saidelectrode tool being divided into a plurality of segments electricallyinsulated from each other and having their end portions complementarilybeveled at angles selected to cause each segment to be aligned in thedirection of roll rotation in overlapping relationship with an adjacentsegment so that the effective edge width of the electrode tool at anypoint along its length is comprised of parts of two adjacent segments;means for positioning said electrode tool parallel to the roll axis withsaid edge adjacent but slightly spaced from the roll working surface;means coupled to the roll and electrode tool for supplying a series ofover-voltage initiated discharges therebetween to remove particles fromthe working surface area of the roll successively least distant from theelectrode surface as the roll rotates, said means for supplyingdischarges including means for supplying discharges between each of saidsegments and said roll; means coupled to said electrode tool for movingsaid tool toward the axis of said roll to maintain a substantiallyconstant gap between the electrode edge and the roll working surface asparticles are removed from the working surface of the roll; and meansfor retaining a dielectric liquid between the electrode edge and theroll working surface to flood said gap.